A common practice in forming fibers of mineral material, such as glass, is to discharge molten glass from a forehearth into a rotating centrifuge or spinner. The molten glass flows across the spinner bottom wall to the spinner peripheral wall and passes in a molten state through the orifices of the spinner peripheral wall to create primary fibers. Thereafter, the primary fibers are usually further attenuated into a veil of secondary fibers of smaller diameter by the action of a downward flow of gases. Hot gases from an annular burner maintain the spinner peripheral wall at a temperature suitable for centrifuging the molten mineral material, and maintain the primary fibers emanating from the spinner in a plastic state to enable further attenuation into secondary fibers. Beneath the spinner, organic binder is sprayed onto the downwardly traveling secondary fibers.
The liquidus temperature of a glass is that temperature below which the molten mineral material begins to crystalize or devitrify. The liquidus temperatures of some high-liquidus glass compositions for glass fibers, such as E Glass, are in the neighborhood of 2000.degree. F. It is well established that centrifugal fiberizing processes are carried out at temperatures higher than the liquidus temperature of the glass in order to avoid crystallization or devitrification of the glass within the spinner. Thus, the effective operating temperatures on the spinner peripheral wall for many high-liquidus glasses are higher than 2000.degree. F. These high operating temperatures involved in fiberizing such high-liquidus materials result in a relatively short spinner life, and result in pollution problems due to the overheating of a portion of the binder as it is sprayed onto the fibers.
It is common to employ borates and other expensive fluxes in centrifuging glass compositions in order to lower the liquidus temperature, and thereby enable fiberization at a lower temperature. For example, in a typical, borate-containing standard wool glass composition, hereinafter disclosed, the liquidus temperature is 1742.degree. F. Using borate fluxes to lower the liquidus temperature is not entirely satisfactory because borates are an expensive commodity being supplied in large part in only two places in the free world. Furthermore, the addition of borates in the glass batch composition creates undesirable borate-derived air pollutants.
Attempts have been made to solve the pollution and material supply problems of the above described processes and compositions. One solution is the use of more sophisticated glass compositions having little or no borate fluxes, thereby enabling operation without harmful air pollutants. Such processes and compositions invariably run hotter than processes using the standard centrifuging compositions having the borate fluxes. There is a need for a method and apparatus for fiberizing molten mineral material at high efficiencies and low temperatures, with a reduction or elimination of borate fluxes in the glass composition.